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Aug 27, 2014 - The VOx/CeO2 catalysts prepared by homogeneous precipitation ... Effect of preparation methods on VOx/CeO2 catalysts for the selective.

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The VOx/CeO2 catalysts prepared by homogeneous precipitation method exhibited high NOx conversion and strong SO2 resistance in NH3-SCR reaction.

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Effect of preparation methods on VO x /CeO 2 catalysts for the selective catalytic reduction of NO x with NH 3 Zhihua Lian, Fudong Liu, Hong He* 5

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x The effect of preparation methods on VO x /CeO 2 catalysts for the selective catalytic reduction of NO x with NH 3 was fully studied. VO x /CeO 2 prepared by a simple homogeneous precipitation method showed higher NH 3 -SCR activity and higher SO 2 and H 2 O resistance than catalysts prepared by other methods. Lower CeO 2 crystallinity on the surface, better dispersion of vanadium species, and higher surface concentration of vanadium species together with more acid sites were all responsible for the higher SCR activity of VO x /CeO 2 prepared by the homogeneous precipitation method. The NH 3 -SCR reaction over VO x /CeO 2 catalysts mainly followed an Eley-Rideal scheme, in which gaseous NO reacted with adsorbed NH 3 species to finally form N 2 and H 2 O.

1. Introduction 15

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Nitrogen oxides (NO and NO 2 ), a major source of air pollution, result from automobile exhaust gas and industrial combustion of fossil fuels.1 They contribute to a variety of environmentally harmful effects such as photochemical smog, acid rain, and haze formation.2 The selective catalytic reduction of NO x with NH 3 (NH 3 -SCR) in the presence of excess oxygen has proved to be the most efficient technology for the removal of nitrogen oxides from stationary and mobile sources.2-3 Many catalysts have been investigated, and V 2 O 5 -WO 3 (MoO 3 )/TiO 2 has been widely applied as an industrial catalyst for many years due to its high catalytic activity and SO 2 resistance.4-5 However, some problems remain for V 2 O 5 -WO 3 (MoO 3 )/TiO 2 , such as the narrow operation temperature window of 300-400 oC, low N 2 selectivity and high conversion of SO 2 to SO 3 at high temperatures.4, 6-7 In addition, the high concentration of ash containing K 2 O, CaO, As 2 O 3 etc. in the flue gas reduces the performance and longevity of V 2 O 5 -WO 3 (MoO 3 )/TiO 2 catalysts in this temperature range.89 Therefore a lot of studies have been performed to develop new NH 3 -SCR catalyst systems or to improve vanadium-based catalysts, especially at low temperatures.10-15 Vanadium-based catalysts with high loading amounts usually exhibited high NH 3 -SCR activity and SO 2 resistance at low temperatures.16-17 For example, V 2 O 5 /AC catalysts were found to State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, Haidian District, Beijing 100085, China. E-mail: [email protected]; Fax: +86 10 62849123; Tel: +86 10 62849123; † Electronic supplementary information (ESI) available: NO x conversion over VO x /CeO 2 with different loading; The N 2 selectivity in NH 3 -SCR reaction; NH 3 -SCR activity after SO 2 poisoning; NH 3 /NO conversion in separate NH 3 or NO oxidation reactions; The band intensity of nitrate species calculated from DRIFTS of VO x /CeO 2 catalysts. This journal is © The Royal Society of Chemistry [year]

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exhibit high catalytic activity in the NO-NH 3 -O 2 reaction at low temperatures.18-20 Therefore, we chose to investigate and optimize vanadium-based catalysts for practical applications. On the other hand, cerium-based catalysts have also been studied extensively due to their high oxygen storage capacity and excellent redox properties, showing high NH 3 -SCR activity in the medium or high temperature ranges. In our previous study, Ce/TiO 2 catalysts have exhibited highly effective NH 3 -SCR activity.21-22 Furthermore, V 2 O 5 /CeO 2 catalysts have also been attracting much attention for their performance in various catalytic reactions. Gu et al.23 have used V 2 O 5 /CeO 2 catalysts for the selective oxidation of toluene, and found that the loading of V 2 O 5 and the calcination temperature influenced the surface structures of dispersed vanadium species as well as the surface acidity and redox properties, which have significant effects on the catalytic activity. In the NH 3 -SCR reaction, a previous study by Li et al.24 showed that V 2 O 5 /CeO 2 catalysts exhibited high NH 3 SCR activity at low temperature, and the NO conversion increased significantly with increasing V 2 O 5 loading. It was reported that V 0.75 Ce oxide catalyst exhibited higher NH 3 -SCR activity than the conventional V 2 O 5 -WO 3 /TiO 2 catalyst below 350 oC.25 Though V-Ce oxide catalysts have shown great catalytic activity, their properties are not well understood and should be investigated in more detail. It is also necessary to decrease the vanadium content due to its toxicity. In many cases, the activity of catalysts is highly dependent on the preparation method. Therefore, in this study we systematically investigated VO x /CeO 2 catalysts in depth, especially the effects of preparation methods on catalyst structure and activity in NH 3 SCR of NO x . Even with low loading content of vanadia, the catalysts could still exhibit excellent catalytic performance for the deNO x process. In addition, the VO x /CeO 2 catalyst prepared by a simple homogeneous precipitation method (VO x /CeO 2 (P)) showed higher NH 3 -SCR activity and better SO 2 resistance than catalysts prepared by other methods, mainly due to lower CeO 2 crystallinity on the surface, better dispersion of vanadium species, and higher surface concentration of vanadium species together with more acid sites. [journal], [year], [vol], 00–00 | 1

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2. Experiments

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2.1 Catalyst synthesis and activity tests

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The 3 wt. % VO x /CeO 2 catalysts were prepared by four methods, including the homogeneous precipitation method, the rotary evaporation impregnation, the incipient wetness impregnation and the sol-gel method. All the materials were purchased from Sinopharm Chemical Reagent Co., Ltd (China) and were analytically pure, except the CeO 2 supports that were prepared by the homogeneous precipitation method as described below. Rotary evaporation impregnation. VO x /CeO 2 was prepared by a rotary evaporation impregnation method using CeO 2 and an aqueous solution of NH 4 VO 3 (H 2 C 2 O 4 was added to facilitate the dissolution of NH 4 VO 3 ). After impregnation, the excess water was removed in a rotary evaporator at 60 oC. The sample was first dried at 100 oC overnight followed by calcination at 500 oC in air for 3 h. The catalyst was denoted as VO x /CeO 2 (V). Homogeneous precipitation method. Aqueous solutions of Ce(NO 3 ) 3 and NH 4 VO 3 were mixed at the required mass ratio (the mass ratio of vanadium oxide was controlled at 3 wt.%). Excess urea in aqueous solution was then added to the mixed solution. The solution was heated to 90 ºC and held there for 12 h under vigorous stirring. After filtration and washing with deionized water, the resulting precipitate was dried at 100 ºC overnight and subsequently calcined at 500 oC for 3 h in air. The VO x /CeO 2 sample prepared by the homogeneous precipitation method was denoted as VO x /CeO 2 (P). Incipient wetness impregnation method. VO x was deposited on CeO 2 by conventional pore volume impregnation with an aqueous solution of NH 4 VO 3 in oxalic acid. After ultrasonic processing for 1 h, the material was dried at 100 oC overnight and calcined at 500 oC for 3 h. The VO x /CeO 2 sample prepared by the incipient wetness impregnation method was denoted as VO x /CeO 2 (I). Sol-gel method. Ce(NO 3 ) 3 , NH 4 VO 3 (at a ratio to yield 3 wt.% vanadium oxide) and excess citric acid were mixed in aqueous solution. The resulting mixture was stirred at room temperature for 1 h. The solution was dried at 120 oC for 12 h, resulting in a porous, foam-like solid. The foam-like precursor was calcined at 500 oC for 3 h in air in a temperature-programmed muffle furnace. The VO x /CeO 2 sample prepared by the sol-gel method was denoted as VO x /CeO 2 (S). Before NH 3 -SCR activity testing, the catalysts were pressed, crushed and sieved to 40-60 mesh. The SCR activity tests were carried out in a fixed-bed quartz flow reactor at atmospheric pressure. The reaction conditions were controlled as follows: 500 ppm NO, 500 ppm NH 3 , 5 vol. % O 2 , 5 vol. % H 2 O (when used), 100 ppm SO 2 (when used), N 2 balance. Under ambient conditions, the total flow rate was 500 ml/min and the gas hourly space velocity (GHSV) was 50 000 h-1. The effluent gas including NO, NH 3 , NO 2 and N 2 O was continuously analyzed by an FTIR spectrometer (Nicole Nexus 670) equipped with a heated, low-volume multiple-path gas cell (2m). The FTIR spectra were collected after the SCR reaction reached a steady state, and the NO x conversion and N 2 selectivity were calculated as follows: 𝑁𝑂𝑥 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 = �1 − 𝑁2 𝑠𝑒𝑙𝑒𝑐𝑡𝑖𝑣𝑖𝑡𝑦 =

[𝑁𝑂]𝑜𝑢𝑡 + [𝑁𝑂2 ]𝑜𝑢𝑡 � × 100% [𝑁𝑂]𝑖𝑛 + [𝑁𝑂2 ]𝑖𝑛

[𝑁𝑂]𝑖𝑛 + [𝑁𝐻3 ]𝑖𝑛 − [𝑁𝑂2 ]𝑜𝑢𝑡 − 2[𝑁2 𝑂]𝑜𝑢𝑡 [𝑁𝑂]𝑖𝑛 + [𝑁𝐻3 ]𝑖𝑛

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DOI: 10.1039/C4CY00935E

2.2 Characterization of catalysts The surface area and pore characterization of the catalysts were obtained from N 2 adsorption/desorption analysis at -196 ºC using a Quantachrome Quadrasorb SI-MP. Prior to the N 2 physisorption, the catalysts were degassed at 300 oC for 5 h. Surface areas were determined by the BET equation in the 0.050.35 partial pressure range. Pore volumes and average pore diameters were determined by the Barrett-Joyner-Halenda (BJH) method from the desorption branches of the isotherms. Powder X-ray diffraction measurements of the catalysts were carried out on a computerized PANalytical X'Pert Pro diffractometer with Cu Kα (λ = 0.15406 nm) radiation. The data of 2θ from 10 to 80o were collected at 8o/min with the step size of 0.07o. Visible Raman spectra of the VO x /CeO 2 catalysts were collected at room temperature on a Spex 1877 D Triplemate spectrometer with spectral resolution of 2 cm−1. A 532 nm DPSS diode-pump solid semiconductor laser was used as the excitation source and the power output was about 40 mW. Before measurements, the catalysts were ground well and mounted in a spinning holder to avoid thermal damage during scanning. The Raman signals were collected with conventional 90o geometry and the time for recording each spectrum was 1000 ms. Raman spectra used in this paper were original and unsmoothed. The H 2 -TPR experiments were carried out on a Micromeritics Auto Chem 2920 chemisorption analyzer. The samples (50 mg) were pretreated at 300 oC in a flow of 20 vol.% O 2 /Ar (50 ml/min) for 0.5 h in a quartz reactor and cooled down to room temperature (30 oC) followed by Ar purging for 0.5 h. A 50 mL/min gas flow of 10% H 2 in Ar was then passed over the samples through a cold trap to the detector. The reduction temperature was raised at 10 oC min-1 from 30 to 1000 oC. X-ray photoelectron spectroscopy (XPS) spectra of the catalysts were recorded on a scanning X-ray microprobe (Axis Ultra, Kratos Analytical Ltd.) using Al Kα radiation (1486.7 eV). All the binding energies were calibrated using the C 1s peak (BE = 284.8 eV) as standard. 2.3 NH 3 -TPD studies

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NH 3 -TPD experiments were performed using a quadrupole mass spectrometer (HPR20, Hiden Analytical Ltd.) to record the signal of NH 3 (m/z = 15 for NH). Prior to TPD experiments, the samples (100 mg) were pretreated at 400 oC in a flow of 20 vol.% O 2 /Ar (50 ml/min) for 0.5 h and cooled down to room temperature (30 oC). The samples were then exposed to a flow of 2500 ppm NH 3 /Ar (50 ml/min) at 30 oC for 1 h, followed by Ar purging for another 1 h. Finally, the temperature was raised to 600 oC in Ar flow at the rate of 10 oC min-1. 2.4 In situ DRIFTS studies

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In situ DRIFTS experiments were performed on an FTIR spectrometer (Nicolet Nexus 670) equipped with a smart collector and an MCT/A detector cooled by liquid nitrogen. The reaction temperature was controlled precisely by an Omega programmable temperature controller. Prior to each experiment, the sample was pretreated at 400 oC for 0.5 h in a flow of 20 vol.% O 2 /N 2 and then cooled down to 200 oC. The background spectra were collected in flowing N 2 and automatically subtracted from the sample spectrum. The reaction conditions were controlled as follows: 300 ml/min total flow rate, 500 ppm NH 3 or/and 500 ppm NO + 5 vol.% O 2 , and N 2 balance. All spectra were recorded by accumulating 100 scans with a resolution of 4 cm-1.

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3.1 Catalytic performance

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3.1.1 SCR activity over VO x /CeO 2 catalysts 5

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The NH 3 -SCR activity over VO x /CeO 2 with different loadings is shown in Figure S1. 3% wt. % VO x /CeO 2 showed much higher catalytic activity than 1%, 0.5%, and 0.1% catalysts, especially at 150-300 oC. Due to the toxicity of vanadium to the human body, a vanadium-based catalyst with too high loading was not preferred. Therefore, we chose the 3% VO x /CeO 2 catalyst to investigate rather than catalysts with higher loading. The NO x conversion over VO x /CeO 2 catalysts prepared by different methods is shown in Figure 1. It is obvious that the preparation methods affected the catalytic activity, especially in the relatively low temperature range. VO x /CeO 2 prepared by the simple homogeneous precipitation method exhibited the best catalytic activity, with nearly 100% NO x conversion and 100% N 2 selectivity at temperatures above 200 oC. VO x /CeO 2 catalysts prepared by rotary evaporation impregnation and incipient wetness impregnation methods showed lower NO x conversion than that prepared by the homogeneous precipitation method, and the catalyst prepared by the sol-gel method showed the lowest catalytic activity. All the catalysts presented higher than 90% N 2 selectivity and only a little N 2 O was produced at the temperature that we investigated (as shown in Figure S2). The preparation methods could affect the structural properties, redox ability and surface acidity of the catalysts, resulting in different catalytic activity, which will be discussed later in this paper.

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Figure 3 shows the effect of SO 2 on the catalytic activity over VO x /CeO 2 catalysts at 250 oC. When 100 ppm SO 2 was introduced to the inlet gas, the NO x conversion over VO x /CeO 2 (S) decreased rapidly to as low as 60% in 24 h and could not recover to the initial activity after the removal of SO 2 The NO x conversion over VO x /CeO 2 (V) and VO x /CeO 2 (I) catalysts also decreased, and after the introduction of SO 2 , the catalytic activity recovered to some extent. However, the SO 2 poisoning behaviour over VO x /CeO 2 (P) was quite different. The NO x conversion decreased slowly, and 93% NO x conversion was obtained in the presence of 100 ppm SO 2 for a 24 h test. VO x /CeO 2 (P) exhibited the highest catalytic activity and the strongest resistance to SO 2 . The NH 3 -SCR performance of VO x /CeO 2 catalysts after SO 2 poisoning for 24 h is shown in Figure S3. The activity over VO x /CeO 2 (P) was still higher than that of other catalysts. 100% NO x conversion could be obtained over the VO x /CeO 2 (P) catalyst at 250 oC and 70% over VO x /CeO 2 (S). This proved again that the VO x /CeO 2 (P) catalyst showed the strongest SO 2 resistance.

Figure 2. NO x conversion over VO x /CeO 2 catalysts in NH 3 -SCR reaction in the presence of H 2 O. Reaction conditions: [NO] = [NH 3 ] = 500 ppm, [H 2 O] = 5 vol. %, [O 2 ] = 5 vol. %, N 2 balance, total flow rate 500 ml/min and GHSV = 50 000 h-1.

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Figure 1. NH 3 -SCR activity over VO x /CeO 2 catalysts prepared by different methods. Reaction conditions: [NO] = [NH 3 ] = 500 ppm, [O 2 ] = 5 vol. %, N 2 balance, total flow rate 500 ml/min and GHSV = 50 000 h-1. 3.1.2 The influence of H 2 O and SO 2 on SCR activity of VO x /CeO 2

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NO x conversion over VO x /CeO 2 catalysts in the NH 3 -SCR reaction with 5 vol. % H 2 O is shown in Figure 2. Compared to NH 3 -SCR activity without H 2 O, NO x conversion in the presence of H 2 O over the four catalysts at low temperatures decreased in all cases to some degree, while catalytic activity at 400 oC increased from 80% to 100%. The NO x conversion over the VO x /CeO 2 (P) catalyst at 200 oC was 80%, and only 20% NO x conversion was obtained over VO x /CeO 2 (S) and VO x /CeO 2 (V). The VO x /CeO 2 (P) catalyst still exhibited the best catalytic performance in the presence of H 2 O.

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Figure 3. Effect of SO 2 on NH 3 -SCR activity over VO x /CeO 2 catalysts at 250 oC.

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Reaction conditions: [NO] = [NH 3 ] = 500 ppm, [SO 2 ] = 100 ppm, [O 2 ] = 5 vol. %, N 2 balance, total flow rate 500 ml/min and GHSV = 50 000 h-1. 3.2 Catalyst characterization

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3. Results

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Catalysts

Surface atomic concentration a (%)

V/Ce atomic

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VO x /CeO 2 (P)

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VO x /CeO 2 (S)

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65.2

0.047

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According to XPS analysis

3.2.1 N 2 physisorption 5

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The surface areas and pore diameters of VO x /CeO 2 catalysts are shown in Table 1. The VO x /CeO 2 (P) catalyst exhibited a slightly higher BET surface area and smaller average pore diameter than VO x /CeO 2 (S), which could offer more active sites for reaction and thus be beneficial to NH 3 -SCR activity. The slight difference in specific surface area indicates that the textural structure is not the crucial factor affecting the catalytic performance.

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Table 1 also shows the surface atomic concentrations of VO x /CeO 2 catalysts derived from XPS results. VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts exhibited similar surface Ce and O concentrations. The surface V concentration of the VO x /CeO 2 (P) catalyst was 2.4%, much higher than that of VO x /CeO 2 (S) (1.6%). The V/Ce atomic ratio was 0.072 and 0.047 for VO x /CeO 2 (P) and VO x /CeO 2 (S), respectively. The higher surface concentration of vanadium species could result in better SCR activity.

3.2.2 XRD

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The XRD patterns of VO x /CeO 2 catalysts are shown in Figure 4. For both catalysts, the only crystalline phase observed was CeO 2 (43-1002). No vanadium species such as V 2 O 5 and CeVO 4 were detected, suggesting that V species were highly dispersed on the catalysts. The intensity of the CeO 2 diffraction peaks of the VO x /CeO 2 (P) catalyst was stronger than that of VO x /CeO 2 (S), indicating that the crystallinity of the CeO 2 phase of VO x /CeO 2 (P) was higher in the bulk phase.

Figure 5. Raman results of VO x /CeO 2 catalysts. 45

3.2.4 H 2 -TPR H 2 -TPR is frequently used to investigate the redox properties of metal oxide catalysts. Figure 6 presents the TPR results of VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts. According to the

Figure 4. XRD patterns of VOx/CeO 2 catalysts 3.2.3 Raman and XPS 25

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The surface-sensitive techniques Raman and XPS were employed for characterization of the VO x /CeO 2 catalysts. Figure 5 shows the Raman results for VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts. In accordance with previous works,22, 26 the Raman shift at 453 cm-1 was attributed to CeO 2 (F 2g mode). No evidence of vanadium-containing phases, such as V 2 O 5 and CeVO 4 , was detected for either catalyst. The CeO 2 peak intensity for the VO x /CeO 2 (P) catalyst was weaker than that for VO x /CeO 2 (S), indicating that the CeO 2 crystallinity on the surface of the VO x /CeO 2 (P) catalyst was weaker. Active sites could thus be better dispersed on the surface of VO x /CeO 2 (P). 4 | Journal Name, [year], [vol], 00–00

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Figure 6. H 2 -TPR results over VO x /CeO 2 catalysts. This journal is © The Royal Society of Chemistry [year]

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Table 1 Surface atomic concentration and BET surface areas and pore diameters of VO x /CeO 2 catalysts.

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acid sites and Lewis acid sites, which was in good agreement with the NH 3 -TPD results. 3.4.2 NO x adsorption 50

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Figure 8(B) shows the DRIFT spectra of NO + O 2 adsorption on VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts at 200 oC. When the VO x /CeO 2 catalyst was exposed to NO + O 2 , several bands assigned to nitrate species were observed. The bands at 1203 and 1592 cm-1 could be assigned to bridging nitrate.34-35 The bands at 1571 and 1245 cm-1 were ascribed to bidentate nitrate,32, 35-36 while the bands at 1502-1542 and 1273 cm-1 were attributed to monodentate nitrate.32, 34 On the VO x /CeO 2 (P) catalyst, the adsorption amount of NO x was larger than that on VO x /CeO 2 (S), especially the amount of monodentate nitrate at 1273 cm-1.

3.3 NH 3 -TPD 20

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Figure 7 shows NH 3 -TPD results over VO x /CeO 2 catalysts using the fragment of m/z = 15 (NH) to identify NH 3 . There were three NH 3 desorption peaks around 90, 250 and 470 oC on both catalysts. The desorption peaks at 90 oC were ascribed to the desorption of physisorbed NH 3 . The broad desorption peaks between 150 and 400 oC were assigned to weak and moderate acid sites on the catalyst surface. With increasing temperature, small peaks between 400 and 600 oC occurred in the NH 3 -TPD profiles, which are related to NH 3 molecules adsorbed on the strong acid sites of the catalysts.31 Though the desorption temperature of VO x /CeO 2 (P) was a litter higher than that of the VO x /CeO 2 (S) catalyst, the amount of NH 3 desorption from the former was notably larger than that of the latter. This indicates that there are more acid sites on the VO x /CeO 2 (P) catalyst.

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Figure 8. DRIFT spectra of 500 ppm NH 3 adsorption (A) and 500ppm NO + 5vol.% O 2 adsorption (B) on VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts. 65

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3.4.3 In situ DRIFTS of the reaction between NO + O 2 and preadsorbed NH 3 species Figure 9(A) shows the in situ DRIFT spectra of the reaction between NO + O 2 and pre-adsorbed NH 3 species on VO x /CeO 2 (P). After NH 3 pre-adsorption and N 2 purging, the VO x /CeO 2 (P) catalyst surface was covered by various NH 3 species. When NO + O 2 was introduced, the intensity of the bands attributed to NH 3 species decreased quickly and disappeared after 5 min. At the same time, bands assigned to nitrate species (monodentate nitrate at 1542, 1273 cm-1, bridging nitrate at 1203, 1597 cm-1 and

Figure 7. NH 3 -TPD results of VO x /CeO 2 catalysts. 3.4 In situ DRIFTS 3.4.1 NH 3 adsorption

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The in situ DRIFT spectra of NH 3 adsorption on VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts at 200 oC are shown in Figure 8(A). After NH 3 adsorption and N 2 purging, both of the catalysts were covered by various NH 3 species. The bands at 1425 cm-1 were assigned to ionic NH 4 + bound to the Brønsted acid sites and the bands at 1594 and 1158 cm-1 were attributed to coordinated NH 3 bound to the Lewis acid sites.32-33 The bands at 1260 cm-1 were assigned to amide species (-NH 2 ).33 The VO x /CeO 2 (P) catalyst exhibited more acid sites than VO x /CeO 2 (S), including Brønsted

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literature,27-28 the TPR peak around 480 oC could be attributed to the reduction of surface Ce4+ to Ce3+. The reduction peak of welldispersed V5+ species could take place at 460 oC.29-30 From Figure 5, the reduction peak of CeO 2 catalyst at 471 oC could be ascribed to the reduction of surface Ce4+ and the peak at 760 oC could be attributed to the reduction of bulk CeO 2 . Over the VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts, the reduction peaks at low temperature showed much higher intensity than that of CeO 2 sample mainly due to the interaction between vanadium and cerium oxides. The H 2 reduction temperature of the VO x /CeO 2 (P) catalyst was lower than that of VO x /CeO 2 (S) in the low temperature region, and the amount of H 2 consumption of the former was larger than that of the latter. In addition, NO/NH 3 oxidation activity over VO x /CeO 2 (P) catalyst was higher than that of VO x /CeO 2 (S) (as shown in Figure S4). This indicates that the redox capability of the VO x /CeO 2 (P) catalyst was a little greater than that of VO x /CeO 2 (S), which could contribute to NH 3 -SCR activity to some degree.

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Figure 9. In situ DRIFT spectra of VO x /CeO 2 (P) (A) and VO x /CeO 2 (S) (B) pretreated by exposure to 500 ppm NH 3 followed by exposure to 500 ppm NO + 5vol.% O 2 at 200 oC.

4. Discussion 4.1 The effect of preparation methods on catalytic activity

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bidentate nitrate at 1571, 1245 cm ) appeared. This result suggested that the adsorbed NH 3 species, including ionic NH 4 + and coordinated NH 3 , could both react with NO x and participate in the NH 3 -SCR reactions. For the VO x /CeO 2 (S) catalyst (Figure 9(B)), similar bands due to NH 3 adsorption were observed after NH 3 pre-adsorption and N 2 purging. After the introduction of NO + O 2 , the adsorbed NH 3 species decreased in intensity, and totally vanished after 10 min, followed by the appearance of nitrate species. The adsorbed NH 3 species on VO x /CeO 2 (S) could also participate in the SCR reaction, similar to VO x /CeO 2 (P).

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3.4.4 In situ DRIFTS of the reaction between NH 3 and preadsorbed NO x species The catalysts were first treated with NO + O 2 for 30 min, followed by N 2 purging. When NH 3 was introduced, the IR spectra were recorded as a function of time. For the VO x /CeO 2 (P) catalyst (Figure 10(A)), after NO + O 2 pre-adsorption and N 2 purging, the catalyst surface was covered by various nitrate species. When NH 3 was introduced, the intensity of the bands attributed to monodentate nitrate and bridging nitrate species decreased slightly. The amount of bidentate nitrate species increased markedly, which may be due to the transformation of monodentate and bridging nitrate to bidentate nitrate. The changes in band intensities of nitrate species on NO x preadsorbed catalysts during the introduction of NH 3 are shown in Figure S5. The bands at 1425 and 1158 cm-1 attributed to adsorbed NH 3 species appeared after NH 3 was introduced. The adsorbed nitrate species could not easily react with adsorbed NH 3 . This suggests that the adsorbed nitrate species were mostly inactive in the NH 3 -SCR reaction.

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The effect of preparation methods for VO x /CeO 2 catalysts was investigated in detail in this study. VO x /CeO 2 prepared by the homogeneous precipitation method showed the highest SCR activity, and nearly 100% NO x conversion plus 100% N 2 selectivity was obtained above 200 oC. In addition, VO x /CeO 2 (P) exhibited the strongest resistance to H 2 O and SO 2 in NH 3 -SCR. Based on the XRD results, the crystallinity of the CeO 2 phase in the VO x /CeO 2 (P) catalyst was higher than that in VO x /CeO 2 (S). However, the Raman spectra showed that the homogeneous precipitation method restrained the crystallization of CeO 2 on the surface layer of the VO x /CeO 2 (P) catalyst. The lower surface crystallinity signifies more defects on the catalyst surface and better dispersion of vanadium species, which could enhance catalytic activity. Higher surface vanadium concentration on the VO x /CeO 2 (P) catalyst, as shown by XPS results, indicated more active sites and improved the NH 3 -SCR performance. In addition, the surface acidity of a catalyst plays an important role in the NH 3 -SCR reaction. VO x /CeO 2 (P) and VO x /CeO 2 (S) showed similar specific surface areas, but the NH 3 desorption amount from the former was much larger, indicating that the VO x /CeO 2 (P) catalyst could provide more acid sites. This could result from the higher surface concentration of vanadium species, since acid sites are more prevalent on vanadium oxides than on cerium oxide. More acid sites on the VO x /CeO 2 (P) catalyst could facilitate the adsorption and activation of NH 3 during the catalytic reaction and thus enhance its catalytic activity in NH 3 SCR. Furthermore, according to the literature37,38, vanadium oxide shows excellent SO 2 resistance in NH 3 -SCR. Therefore, a higher surface concentration of vanadium species on the catalyst surface could enhance SO 2 resistance. The VO x /CeO 2 (P) catalyst showed higher catalytic activity in the presence of 100 ppm SO 2 than that of VO x /CeO 2 (S). 4.2 SCR reaction mechanism

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Figure 10. In situ DRIFT spectra of VO x /CeO 2 (P) (A) and VO x /CeO 2 (S)(B) pretreated by exposure to 500 ppm NO + 5vol.% O 2 followed by exposure to 500 ppm NH 3 at 200 oC. For the VO x /CeO 2 (S) catalyst (Figure 10(B)), similar bands ascribed to nitrate species were observed after NO + O 2 adsorption and N 2 purging. When NH 3 was introduced, the intensity of bridging nitrate decreased slowly and the bands attributed to monodentate and bidentate nitrate species stayed unchanged. Adsorbed NH 3 species began to form on the VO x /CeO 2 (S) catalyst surface after 2 min upon NH 3 introduction. The adsorbed nitrate could not easily take part in the NH 3 -SCR reaction.

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NH 3 could adsorb on VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts to form NH 4 + and coordinated NH 3 . When NO x was introduced, NH 3 adsorbed species disappeared quickly. Both NH 4 + and coordinated NH 3 could react with NO x . Monodentate, bridging and bidentate nitrates were deposited on the catalyst surface when NO + O 2 were introduced. Adsorbed nitrate species were mostly inactive in the NH 3 -SCR reaction. After NH 3 was introduced, the number of monodentate and bridging nitrates reduced slightly, but the amount of bidentate nitrate increased. The gaseous NO mainly interacted with adsorbed NH 3 species on VO x /CeO 2 catalysts to form an activated intermediate, and subsequently decomposed to N 2 and H 2 O according to an Eley-Rideal scheme.39

5. Conclusions

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VO x /CeO 2 catalysts exhibited excellent NH 3 -SCR performance. VO x /CeO 2 prepared by a simple homogeneous precipitation method showed higher catalytic activity and better H 2 O and SO 2 resistance than catalysts prepared by other methods. As high as 93% NO x conversion was obtained in the presence of 100 ppm SO 2 for a 24 h test over the VO x /CeO 2 (P) catalyst. The weaker crystallinity of CeO 2 in the surface layers of VO x /CeO 2 (P) implied more defects on its surface and better dispersion of vanadium species than that on VO x /CeO 2 (S).The higher surface vanadium concentration led to more acid sites on This journal is © The Royal Society of Chemistry [year]

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VO x /CeO 2 (P), which can absorb and activate more NH 3 species. All these factors contribute to the higher SCR activity and SO 2 resistance of VO x /CeO 2 (P). The NH 3 -SCR reaction over VO x /CeO 2 (P) and VO x /CeO 2 (S) catalysts mainly followed an Eley-Rideal scheme, in which gaseous NO reacted with adsorbed NH 3 species to finally form N 2 and H 2 O.

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This work was financially supported by the National Science Foundation of China (51108446) and the Ministry of Science and Technology, China (2012AA062506, 2013AA065301). 80

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